1. Field of the Invention
This invention relates to laser shock peening of hard metallic gas turbine engine parts and, more particularly, for using low energy lasers and small laser spots to laser shock peen portions of objects such as airfoil leading and trailing edges such as found on fan and compressor blades to form laser shock peened regions having localized compressive residual stresses imparted by laser shock peening.
2. Description of Related Art
Gas turbine engines and, in particular, aircraft gas turbine engines rotors operate at high rotational speeds that produce high tensile and vibratory stress fields within the blade and make the fan blades susceptible to foreign object damage (FOD). Vibrations may also be caused by vane wakes and inlet pressure distortions as well as other aerodynamic phenomena. This FOD causes nicks and tears and hence stress concentrations in leading and trailing edges of fan blade airfoils. These nicks and tears become the source of high stress concentrations or stress risers and severely limit the life of these blades due to High Cycle Fatigue (HCF) from vibratory stresses. These and other operational phenomena lead to incipient cracking and material failure of portions of objects such as along airfoil edges.
Therefore, it is highly desirable to design and construct longer lasting fan and compressor blades, as well as other hard metallic parts, that are better able to resist both low and high cycle fatigue and that can arrest cracks than present day parts. The above referenced U.S. Patent Applications are directed towards this end. They teach to provide an airfoil of a fan blade with regions of deep compressive residual stresses imparted by laser shock peening on at least a radially extending portion of leading and/or trailing edge surfaces of the fan blade.
The region of deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and thereby harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled "Method and apparatus for truing or straightening out of true work pieces". The prior art teaches the use of multiple radiation pulses from high powered pulsed lasers and large laser spot diameters of about 1 cm to produce shock waves on the surface of a work piece similar to the above referenced Patent Applications and U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser shock processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser shock peening as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". Manufacturing costs of the laser shock peening process is a great area of concern because startup and operation costs can be very expensive. The "on the fly" laser shock peening process disclosed in Ser. No. 08/362,362, pending, above is designed to provide cost saving methods for laser shock peening as is the present invention. However, this prior art teaches to use large laser spots, on the order of 1 cm and greater in diameter, and high powered lasers. Manufacturers are constantly seeking methods to reduce the time, cost, and complexity of such processes and it is to this end that the present invention is directed.